1. FIELD OF THE INVENTION
This invention relates to exposure-control apparatus for use in a camera and more particularly to the use of a single electromagnet in a light-regulated circuit to control adjustment of a diaphragm and a shutter in relation to the level of scene brightness.
2. DESCRIPTION OF THE PRIOR ART
Camera exposure-control apparatus often use a light-responsive, electromagnetic circuit to control operation of a diaphragm and shutter in relation to scene brightness. Conventionally, the circuit uses two electromagnets, both initially energized. One electromagnet controls adjustment of the diaphragm and the other electromagnet controls closure of the shutter. A light-level measuring subcircuit comprising a photosensor and series-connected resistance is most often used for controlling the one electromagnet to adjust the diaphragm, and a light dependent R-C subcircuit comprising a photosensor and series-connected capacitance is used to control the other electromagnet to close the shutter. It has also been proposed to use a single electromagnet in conjunction with such circuit to control both the diaphragm and shutter, (See U.S. Pat. No. 3,385,187). The single electromagnet is coupled with the light-level measuring subcircuit and is initially energized to retain a locking pawl during the time when a diaphragm, in the form of a Waterhouse wheel, operates to vary an exposure aperture; the electromagnet becomes deenergized to release the pawl to lock the diaphragm at a selected aperture size related to scene brightness. The action of locking the diaphragm also locks a shutter blade to prevent the blade from moving to close the exposure aperture. The electromagnet is then coupled with the light dependent R-C circuit. At the end of an exposure interval as established by the R-C circuit, the electromagnet becomes re-energized and re-attracts the locking pawl to release the shutter blade so that it moves to close the exposure aperture. Thus, the electromagnet becomes de-energized to set the diaphragm and subsequently becomes re-energized to initiate shutter closing. When the electromagnet becomes energized to remove the locking pawl from its locking position, the electromagnet force must (1) act through an air gap, (2) act against the spring force holding the pawl in its locking position and (3) act against the mass of the locking pawl. The electromagnetic force required for such operation is high and requires a camera battery of high current capacity to reliably operate for long periods. To reduce the electromagnetic force and its attendant current drain on a camera battery, light spring forces are employed to hold the locking pawl in position. However, such light spring forces do not reliably lock the diaphragm or shutter; thus the use of the electromagnet in this manner to control the diaphragm and shutter does not provide effective use of a single electromagnet to control both diaphragm and shutter.